A Review of Relay selection based Cooperative Wireless Network for Capacity Enhancement

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
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A Review of Relay selection based Cooperative Wireless Network for
Capacity Enhancement
Amrita Dubey1, Anandvardhan Bhalla2
1PG Scholar in Digital Communication, Ojeswini Institute of Management & Technology, Damoh (MP), India
2Asst. Professor, Dept. Of Electronics Communication, Ojeswini Institute of Management & Technology, Damoh
(MP), India
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Abstract - Cooperative communication can be considered
as a promising technology to significantly increase the
transmission rate along with the coverage area of a wireless
network which aims to achieve spatial diversity via the
cooperation of user terminals in transmission without
requiring installation of multiple transceiver antennas. The
user terminals which can be used for this purpose can be
termed as relay nodes. However, most of the existing solutions
on relay node assignment problem with multiple source-
destination pairs are limited to assign each singlepairatmost
one cooperative relay node. Therefore, this paper studies the
cooperative relay node assignment problem in a network
environment, wheremultiple source-destinationpairscompete
for the same pool of cooperative relay nodes andeachpaircan
be allotted multiple cooperative relay nodes for cooperative
communication to maximize the transmission rate and hence
improves the performance among all communicating pairs.
Without requiring multiple antennas on the same device,
spatial diversity is achieved by exploiting the antennas on
other nodes, i.e., relay nodes, in the network. Therefore, the
selection of relay nodes has a significant impact on the
achieved total capacity.
Key Words: Cooperative communication, MRC, DSTC,
DTSBC, AF, DF etc...
1. INTRODUCTION
In the past decade, there has been a tremendous growth in
the area of telecommunications. As cellular phones and the
high speed Internet become more and more popular, the
demand for faster and more efficient telecommunication
systems has been skyrocketing. Both the increase in the
number of users and the increase in high data rate transfers
from the Internet have produced a hugeincreaseintraffic. In
order to handle this heavy traffic, the whole
telecommunications infrastructure has been transformed in
the past several years. Thousands and thousands of optical
fibers were laid underground to allow high speed, wide
bandwidth signal transfer. This has solved most of the
problems for land-based systems. However, more and more
high-speed services are now carried out in a mobile
environment where data transfer is done through wireless
channels. Compared to the capacity of a fiber, the capacity of
a wireless link is the weakest part in the whole
infrastructure. Many people haverealizedthisandhavetried
different methods to increase the bandwidth of the wireless
channels. Before going any further, let us explain how
wireless communications systems work [1].
In most wireless communications systems, there are two
major components - base stations and the mobiles. The base
station is located at the center of a coverage area called a
‘cell’ and the mobiles can be anywhere within the cell.
Communication then takes place between the base station
and the mobile through the wireless channel. A certain
amount of spectrum is assigned to a cell for signal transfer.
This spectrum provides the media for the signals to be
transmitted. With a wider spectrum, more users can be
served within a cell. To serve a large area, one can use a high
power base station to cover the whole area. But only a fixed
amount of users can be served in this way. So instead of
having a single large cell, multiple cells with smaller size are
usually used to cover a large area. To avoid severe
interference between cells, base stations thatareadjacent to
each other are allocated different spectrum or channel
groups. Also the output power of the base station is
maintained at a level so it is just enough to cover the whole
area up to the cell boundary. With this method, cells that are
separated by a certain amount of distance can use the same
spectrum. This is true as long as the interference caused by
surrounding co-channel cells is within tolerable limits. In
this frequency reuse scheme, each cell in theclusterhasone-
seventh of the total spectrum, andthesamespectrumisused
in cells with the same label. With this cellular reuse method,
the same spectrum can now be reused over and over again
and can be used to cover an infinitely large area.
2. BACKGROUND
As mentioned earlier, Cooperative Communications help in
improvingthesystemreliability.CooperativeDiversitycanbe
defined as a form of space diversity [9]. Some of the popular
cooperatives techniques are Amplify and Forward and
Decode and Forward [10-12]. In the former method, the
relays receive the information, amplify the signal and
forwards again to the destination, whereas in the later
method, the relays completely decode the original signal,
encode again and then transmit to the destination. Since the

relays also send the original signal, these methods are called
repetition based cooperative algorithms. It is also clear that
this method causes a decrease in bandwidth, as the nodes
require separate channels within the limited bandwidth.
Distributive Space Time Coding (DSTC) [13] can be used to
realize cooperative diversity to prevent the bandwidth
limitations. As mentionedin[14],DSTBCisadistributedform
of STBC i.e., a replica of the information is shared among the
cooperating nodes for transmission. Since we are dealing
with DSTBC and cooperative diversity, we consider a single
relay system and emphasize these methods.
Fig. 1: Single channel based cooperative communication
using DSTC [15]
A single channel based cooperative communication using
DSTC is shown in fig 1. The system consists of a sender, a
relay and a destination. During the first time slot, the
transmitter sends two symbols, s(n) and s(n+1) to the relay
(* denotes conjugate of the symbol, α1 and α2 are the real
coefficients which are related as 12
2
2
1  [15], [16]).
During the second time slot, the sender and relay
cooperatively transmit the blocks. F and G are the coding
matrices used to encode the signals. These space-time
encoding matrices are orthogonal in nature. Hence they can
be transmitted by the sender and relay at the same time and
thereby improving the reliability of the communication as
well [15].
2.1 Amplify and Forward
The standard method of communication is transmitting the
information from sender to the receiver. In cooperative
communication, the sender sends the same information to
both relay and the receiver during the first time slot. In the
next timeslot, the destination receives another setofsignals;
one from the sender and the other is an amplified version of
the signal in the first time slot. This amplified signal is
transmitted by the relay. Now, the receiver can decode the
combined signal using Maximum Ratio Combining (MRC).
This can be performed with the help of a matched filter. The
relay receives the information signal appended by the
channel gain and noise.
It is then amplified and sent to the destination. The final
information received is a combination of the input signal
convolved with the channel gain and Additive White
Gaussian Noise (AWGN). It is easy to implement because
there is no possibility for errors while decoding which could
be introduced in decode and forward. The major
disadvantage of this method is that the noise will also be
amplified at the relay. This can be eliminated bymaintaining
a high threshold level at the reception. Amplify and forward
method is more advantageous than decode and forward
method in achieving maximum diversity. Mathematically, it
can be shown that, the outage probability is inversely
proportional to the square of theSignal-to-Noiseratio(SNR).
Hence, an increase of 10dB SNR can reduce the outage
probability by an order of 100 [17].
Fig.2: Schematic representation of Amplify and Forward
[18]
2.2 Decode and Forward
In this method, the relay decodes the received signal from
the sender, checks for errors and re-encodes to transmit to
the destination. The receiver estimates the information
signal using Maximum Ratio Combining.The relaycaneither
decode the entire signal or it can performsymbol-by-symbol
decoding [19]. When the relay decodes the received signal,
the noise introduced in the information signal should be
removed prior to decoding. Otherwise, it is more likely that
the original signal could be corrupted.
Fig.3: Schematic representationofDecodeandForward[18]

The destination must decode the signal completely. Hence
the destination must be aware of both the decoding
techniques in order to fully decode the signal. In real time
environments, there is less certainty that the decodedsignal
sent from the relay would again be noise affected. Nicholas
Laneman et. al. have shown in [17] that the decode and
forward method fails to attain full spatial diversity.
Mathematically, they proved that the outage probability is
inversely proportional to the Signal-to-Noise ratio. i.e., the
outage probability reduces at the same as the SNR rises.
3. CHALLENGES IN WIRELESS COMMUNICATION
A fundamental challenge of the wireless communication is
the low reliability of the wireless channels. These pertain to
the channel impairment due to multipath, fading, and
shadowing, which is caused by receiving different versions
of the source signal from different paths. The propagate
paths result from scattering, reflection and diffraction of the
transmitted signals by objects in the environment [2], such
as buildings, trees, etc. Therefore, the capacity of a wireless
channel has very high variability.
In addition to the unreliable channel, the spectrum resource
over wireless channel is limited. Meanwhile, the required
spectrum is increased as a consequence of high demand of
high data rate services. Therefore, in order to reuse the
frequency resource and satisfy the QoS requirements, the
cellular coverage is restricted. Moreover, the increasing
demand of high data rate transmission and the migration to
high carrier frequency regions greatly limits the coverage of
the wireless network. With the potential use of millimeter
wave band in next generation wirelesssystems[3,4],the cell
size is reduced because by the Friis free-space equation, a
millimeter wave signal experiences tens of dB more
attenuation than a microwave signal. On other hand, to
improve the transmission bit rate, more spectrums is
required, and it also leads to coverage reduction. For
instance, the coverage area has to reduce by 13 times, if the
corresponding transmission bit rate rises by 100 times [5].
The natural method to address the coverage issue is to
increase the number of the base stations in a region.
However, it significantly raises the infrastructure cost.
Furthermore, energy consumption and environmental issue
are caused by the fast growing data traffic volume and
remarkable expansion of network infrastructures. From[6],
approximately 18% of the Operation Expenditure results
from the energy bill in the mature European market and at
least 32% in India. Meanwhile, the enormous escalation of
energy consumption due to the explosive development of
wireless communications will directly result in the increase
of greenhouse gas emission and becomes one of the major
challenges in meeting the cost reduction and green
environment targets [7].
Many techniques of wireless communications are emerging
that suggest different architectures for the described
problems above. Cooperative communications as an
attractive alternative is suggested by the standard task
group in IEEE 802.16j Mobile Multihop Relay to extend the
coverage of a base station by deployingseveral relaystations
around the base station [8].
4. PROPOSED WORK
Cooperative communications, shown in Figure 4, have
recently become a key technology for the modern wireless
networks as an effective means of saving power, attaining
broader coverage range, and mitigating channel
impairments resulting from fading. It has been incorporated
into many wireless standards, such as 3GPP long-term
evolution (LTE) [20-22], IEEE 802.11s (mesh networking)
[23], IEEE 802.16j (wireless multihop relay) [19], IEEE
802.16m (WiMAX2) [24, 25] and Femtocell [26].
Fig. 4: Cooperative communications in a wireless network.
Relay communication, which is a specific kind of wireless
cooperative communication, hasbeendemonstratedtobe an
effective way to combat wireless fading by providing spatial
diversity without the need of multi-antenna configurations
[27]. The fundamental idea of relay communication is that
several relay terminals participate in communications by
retransmitting a signal from a source to a destination by
forming a distributed multi-antenna system [28, 29].
Relaying mimics MIMO communications by establishing
interactions among the distributed nodes that serve as
virtual multiple antennas both at the transmitter and
receiver sides. The final destination receives multiple
versions of the transmitted signals through the cooperative
relay nodes and combines them forming the final received
signal. In relay systems, the received and retransmitted
signals at any relay node are typically orthogonalized to
avoid the interference.
It means the transmissions of source-relay and relay-
destination are performed in different time slots or
frequency bandwidths. The application of relays in cellular
systems can permit economical designforthecasethatthere
is few or infrequent user at the edge of cellular. Since the
relay does not need a wired connection to the backhaul, it
can eliminate the costs of the backplane that serves as the
interface between the BS and the wired backhaul network.
Besides, relay communications will reduce the required
transmit power compared to those for a base station (BS)
due to the smaller coverage. Moreover, if the density of

relays in a cell is moderately high, the propagation loss from
the relay to a terminal is much lower than from a BS to the
terminal. Thereby higher data ratescanbeachievedinlarger
cells [30].
Furthermore, multi-hop relay cooperation scheme is an
attractive approach to solve the area coverageproblem with
a reasonable infrastructure cost. In a cellular, all idle users
can act as relays to participate in the communication by
establishing independent paths between the source and the
destination. Hence, there is a potentially large number of
relays which are able to help the base station to forward the
signals. When the target mobile station is out of the service
coverage of the base station, one or more relay node can
retransmit the signals from the base station towards the
destination. As a result, the coverage is substantially
extended with the assist of relay nodes.
5. CONCLUSION
The Relay choice plays an important role in increasing the
diversity gain achieved in wireless cooperative
communication systems, and therefore it has mostly
attracted attention of the many research teamsoverthepast
few years. Different relay choice schemes are designed with
completely different focus and objectives. during this paper,
we have a tendency to mentioned a literature survey on a
number of the foremost representative areas of relay
selection/assignment domain in cooperative networks on
with varied types of relaying waysandtechniques.Designing
the reliable relay assignment scheme may be a principle
working area in wireless cooperative networks. The
implementation of the relay nodes with intelligence to
perform some constant operations (such as compress,
demodulate, amplify etc.) on the incomingsignalsandbrings
out the advantageous manipulations within their behaviour
in the cooperative surroundings will become important
research area. We also centered on the optimum relay
assignment algorithm that provides the optimum answer to
allocate the relay node for enhancing the capability. in an
exceedingly later a part of the article, a ray of light on the
MIMO technology shows the foregone conclusion of this
technology within the area of cooperative communication in
relaying network.
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A Review of Relay selection based Cooperative Wireless Network for Capacity Enhancement